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Thermal performance, economic and environmental life cycle analysis of thermosiphon solar water heaters
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Solar Energy 83 (2009) 39–48 www.elsevier.com/locate/solener
Thermal performance, economic and environmental life cycle analysis of thermosiphon solar water heaters Soteris Kalogirou * Cyprus University of Technology, Department of Mechanical Engineering and Materials Sciences and Engineering, P.O. Box 50329, Lemesos 3603, Cyprus Received 29 January 2007; received in revised form 5 November 2007; accepted 23 June 2008 Available online 18 July 2008 Communicated by: Associate Editor Dr. D. Yogi Goswami
Abstract In this paper, the environmental benefits or renewable energy systems are initially presented followed by a study of the thermal performance, economics and environmental protection offered by thermosiphon solar water heating systems. The system investigated is of the domestic size, suitable to satisfy most of the hot water needs of a family of four persons. The results presented in this paper show that considerable percentage of the hot water needs of the family are covered with solar energy. This is expressed as the solar contribution and its annual value is 79%. Additionally, the system investigated give positive and very promising financial characteristics with payback time of 2.7 years and life cycle savings of 2240 € with electricity backup and payback time of 4.5 years and life cycle savings of 1056 € with diesel backup. From the results it can also be shown that by using solar energy considerable amounts of greenhouse polluting gasses are avoided. The saving, compared to a conventional system, is about 70% for electricity or diesel backup. With respect to life cycle assessment of the systems, the energy spent for the manufacture and installation of the solar systems is recouped in about 13 months, whereas the payback time with respect to emissions produced from the embodied energy required for the manufacture and installation of the systems varies from a few months to 3.2 years according to the fuel and the particular pollutant considered. It can therefore be concluded that thermosiphon solar water hearting systems offer significant protection to the environment and should be employed whenever possible in order to achieve a sustainable future. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Thermosiphon; Solar water heating; Thermal performance; Economic analysis; Embodied energy; Environmental life cycle analysis
1. Introduction All nations of the world depend on fossil fuels for their energy needs. However, the obligation to reduce CO2 and other gaseous emissions, in order to be in conformity with the Kyoto agreement is the reason behind which countries turn to non-polluting renewable energy sources. In developed countries, energy consumption in the building sector represents a major part of the total energy budget. In the European Union this is approximately equal *
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to 40% of the total energy consumption (Argiriou et al., 1997). A considerable percentage of this amount is spent for hot water production. One way to reduce this amount of energy is to employ solar energy. In literature, there are numerous studies on the environmental life cycle analysis of a variety of thermal systems. Some of them deal with solar water heating systems (Mirasgedis et al., 1996; Taborianski and Prado, 2004; Tsillingirides et al., 2004). Particularly, the latter study examined the life cycle environmental impact of a thermosiphon domestic solar hot water system in comparison with electrical and gas heating in Greece. In some other studies only the economic life cycle savings are reported (Hasan, 2000; Keyanpour-Rad et al., 2000).
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S. Kalogirou / Solar Energy 83 (2009) 39–48
Nomenclature A b0 C Ca Caux c0 c1 c2 Cf CFA CFL CFC Cload Cm Cs d f FR Gt i kas
collector area (m2) constant of incident angle modifier equation investment cost (€) collector area dependent cost (€/m2) cost of auxiliary energy (€) intercept efficiency negative of the first-order coefficient of the efficiency (W/m2 °C) negative of the second-order coefficient of the efficiency (W/m2 °C2) collector area independent cost (€) cost rate of auxiliary energy (€/kJ) cost rate of conventional fuel (€/kJ) chlorofluorocarbons cost of fuel to cover load (€) maintenance cost (€) collector investment cost (€) market discount rate (%) solar contribution heat removal factor total global solar radiation (W/m2) interest rate (%) incidence angle modifier
In this paper, initially the environmental benefits of renewable energy systems are shortly discussed, followed by the analysis of the thermal performance, economics and environmental benefits resulting from the use of thermosiphon solar water heating systems. Additionally, the amount of pollution saved because of the use of solar energy against the pollution caused for the manufacture of the systems is examined. 2. Environmental benefits of renewable energy technologies Renewable energy technologies produce marketable energy by converting natural phenomena into useful forms of energy. These technologies use the sun’s energy and its direct and indirect effects on the earth as the resources from which energy is produced. These resources have massive energy potential, however, they are generally diffused and not fully accessible, most of them are intermittent, and have distinct regional variabilities. These characteristics give rise to difficult, but solvable, technical and economical challenges. Today, significant progress is made by improving the collection and conversion efficiencies, lowering the initial and maintenance costs and increasing the reliability and applicability of renewable energy systems (Kalogirou, 2004b). Two potential solutions to the current environmental problems associated with the harmful pollutant emissions from the burning of fossil fuels are renewable energy and
LCS N g PW Qins Qenv Qu Qaux Qload Qloss Ta Ti TMY UL UV VOC
life cycle savings (€) number of years collector efficiency present worth (€) total radiation incident on the collector (kJ) heat losses from storage tank (kJ) useful energy supplied form solar collectors (kJ) auxiliary energy (kJ) energy required to cover load (kJ) the annual energy lost from the storage tank and pipes (kJ) ambient temperature (°C) inlet temperature to the collector (°C) typical meteorological year heat loss coefficient (W/m2 °C) ultraviolet volatile organic compound
Greek symbols DT temperature difference [=Ti Ta] (°C) h incidence angle (degrees) sa transmittance–absorptance product
energy conservation technologies. Many countries consider today solar, wind and other renewable energy technologies as the key to a clean energy future. Renewable energy systems can have a beneficial impact on the environmental, economic, and political issues of the world (Kalogirou, 2004b). The benefits arising from the installation and operation of renewable energy systems can be distinguished into three categories: energy saving, generation of new working posts and the decrease of environmental pollution (Diakoulaki et al., 2001). The most important benefit of renewable energy systems is the decrease of environmental pollution. This is achieved by the reduction of air emissions due to the substitution of electricity and conventional fuels. The most important effects of air pollutants on the human and natural environment are their impact on the public health, agriculture, buildings and historical monuments, as well as on forests and ecosystems (Diakoulaki et al., 2001). It is relatively simple to measure the financial impact of these effects when they affect tradable goods such as the agricultural crops; but this is much more complicated when it comes to non-tradable goods like human health and ecosystems. This paper deals with thermosiphon solar water heating systems, which are very popular systems, used extensively in many countries with good sunshine potential such as the Mediterranean countries. A domestic size system is considered, which is analysed with respect to its
S. Kalogirou / Solar Energy 83 (2009) 39–48
energy performance, impact.
economics
and
environmental
3. Thermosiphon solar water heating systems Thermosiphon, or natural circulation, solar water heating systems (also called passive systems) are the simplest and most widely used solar energy collection and utilization devices (Fig. 1). They are intended to supply hot water for domestic use, and are based on natural circulation or thermosiphon principle. They supply hot water at a temperature of about 60 °C and consist of a collector, storage tank, and connecting pipes. A schematic diagram of the thermosiphon systems is shown in Fig. 1a. These systems heat potable water or a heat transfer fluid and use natural convection to transport it from the collector to storage. Thermosiphoning occurs when the water in the collector expands becoming less dense as heat is added by solar energy and rises through the collector header into the top of the storage tank. There it is replaced by the cooler water that has sunk to the bottom of the tank from which it flows down the collector. Circulation continuous as long as the sun is shining. Since the driving force is only a small density difference between the hot and cold water, larger than normal pipe sizes must be used to minimize pipe friction. In the storage tank, hot water accumulates near the top when water is heated during the day by solar radiation. To take into account periods of low solar radiation levels, storage tanks are normally sized to hold about two days’ supply of hot water. It should be noted that the water flowing through the collectors is potable water that goes to the user and any quantity of hot water used is replaced through the freshwater inlet (from the cold water storage tank or mains supply) which enters the storage tank near the bottom so as not to break the stratification.
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Connecting lines must be well insulated to prevent heat losses and sloped to prevent formation of air pockets which would stop circulation. At night, or whenever the collector is cooler than the water in the tank the direction of the thermosiphon flow will reverse, thus cooling the stored water, unless the top of the collector is placed well below (about 30 cm) the bottom of the storage tank (Kalogirou, 2004a). The size of a thermosiphon solar system depends on the prevailing weather conditions and the hot water requirements. The collector area is determined primarily by the daily hot water demand, which varies from place to place depending on local customs and lifestyles and is normally about 30 l/person/day. A typical unit operating in a good environment (Mediterranean area) usually consists of two flat-plate solar collectors having an absorber area between 2.5 and 4 m2, and a storage tank with capacity between 150 and 180 l. An auxiliary electric immersion heater and/or a heat exchanger, for central heating assisted hot water production, are used in winter during periods of low solar insolation. Such a system covers about 80% of the hot water requirements of a four-person family. The flat-plate collector is generally fixed permanently in position, and therefore the tilt of the collector is determined primarily by consideration of the predominant season of hot water use. For year-round use, the collector tilt is kept equal to the latitude of the location plus 5°. In the northern hemisphere, the collector faces direct south (azimuth angle = 0°), although a shift of a few degrees towards east or west does not greatly influence its performance (Kalogirou, 2004a). The daily overall system efficiency of a domestic solar hot water system is about 30–40%, and the temperature difference between the collector outlet and inlet is about 10 °C. The storage tank is placed horizontally or vertically. Though the shallow depth of the horizontal tanks degrades stratification, for horizontal tanks with
Mixing device Storage tank
Hot water outlet
Auxiliary Cold water inlet
Collector
(a) Schematic diagram
(b) Photograph
Fig. 1. Schematic diagram and a photograph of a thermosiphon solar water heater.
S. Kalogirou / Solar Energy 83 (2009) 39–48
diameters greater than 500 mm there is only a small performance loss in comparison with vertical tanks. The main disadvantage of thermosiphon systems is the fact that they are comparatively tall units, which makes them not very attractive aesthetically. Usually, a cold water storage tank is installed on top of the solar collector, supplying both the hot water cylinder and the cold water needs of the house, thus making the collector unit taller and even less attractive (see Fig. 1b). Additionally, extremely hard or acidic water can cause scale deposits that may clog or corrode the absorber fluid passages. For direct systems, pressure-reducing valves are required when the city water is used directly and pressure is greater than the working pressure of the collectors. The unit considered in this work employs flat-plate collectors which are by far the most used type of collector. The instantaneous efficiency of the collector considered is given by the following equation obtained by testing the collector at the Applied Energy Centre of the Ministry of Commerce and Industry, Nicosia, Cyprus: 2 DT DT 0:06 ð1Þ n ¼ 0:792 6:65 Gt Gt where DT is temperature difference between the collector inlet (Ti) and ambient (Ta) temperatures, i.e., DT = (Ti Ta), and Gt is the global solar radiation. The storage tank is well insulated to reduce thermal loses to the environment and is equipped with heat exchangers for heating the water with auxiliary energy. The auxiliary can be either electricity or diesel. In the case where diesel is considered this is used in a central heating boiler, which supplies the energy for the heating needs of a house and is not used only as the solar system backup. What is of interest to note is that if the temperature of the water in the storage tank is more than the desired temperature this is mixed with the make-up water to obtain the required temperature. This is done at the tap by the user (through the mixer) but in the simulation, it is done with the mixing devise shown in Fig. 1a. The specifications of the various components of the solar system are shown in Table 1. With regard to the thermal load, although the hot water demand is subject to a high degree of variation from day to day and from consumer to consumer it is impractical to use anything but a repetitive load profile. This is not quite correct during the summer period, where the consumption Table 1 Specifications of the thermosiphon solar water system considered Parameter
Domestic hot water system 2
Collector area (m ) Collector slope (°) Storage capacity (l) Auxiliary capacity (kW) Heat exchanger Heat exchanger area (m2) Hot water demand (l)
2.7 (2 panels) 40 150 3 Internal 3.6 120 (4 persons)
Hot water consumption (l)
42
15 10 5 0 0
2
4
6
8
10 12
14
16
18
20
22
24
Hours
Fig. 2. Hot water daily consumption profile.
pattern is somewhat higher. However, during this period, the temperature requirement for hot water is not as high as during winter. Consequently, the total thermal energy requirement is reasonably constant throughout the year. For the present simulation, the hot water consumption profile illustrated in Fig. 2 is employed, which assumes a daily hot water consumption of 120 l at 50 °C for a family of four (30 l/person). Traditional hot water systems comprise a hot water cylinder powered either by electricity or by diesel oil through the central heating boiler. Therefore, the extra equipments required for the solar system are the solar collectors and the piping to connect the collectors with the storage tank. 4. Thermal and economic analysis of the solar system The system is modelled with the well-known TRNSYS simulation program. TRNSYS is employing the standard second-order collector performance equation to model the collector, given by (TRNSYS, 2005) n ¼ c0 K h c1
ðT i T a Þ ðT i T a Þ c2 Gt Gt
2
where Kh is the incidence angle modifier given by 1 1 K h ¼ 1 b0 cosðhÞ
ð2Þ
ð3Þ
The values of c0, c1, c2 and b0 are obtained by experimental testing of collectors in accredited laboratories. For the present application the factors shown in Eq. (1) are used, for c0, c1 and c2, whereas b0 is equal to 0.1, also determined from testing the system in the test centre described above. The useful energy extracted from the collectors is given by Qu ¼ F R A½Gt ðsaÞK h U L ðT i T a Þ
ð4Þ
The total useful energy for the whole year is obtained from Qu;a ¼
365 X 24 X
Qu
ð5Þ
d¼1 h¼1
and the auxiliary energy required, Qaux is Qaux ¼ Qload ½Qu;a Qloss
ð6Þ
where Qload is the energy required by the load and Qloss is the energy lost from the storage tank and pipes.
S. Kalogirou / Solar Energy 83 (2009) 39–48
As can be seen from the above equations the energy obtained from the solar collector field depends on the collector area (A), collector slope (affects cosh), flow rate (affects FR) and the storage tank size (affects Ti). The collector inlet temperature depends also on the load pattern, make-up water temperature and the losses from the storage tank and pipes. The storage tank losses depend on the temperature of the stored water, i.e., it depends on the energy collected and storage tank size. The main component that models the thermosiphon system is TYPE 45. This component models a system consisting from a flat-plate solar collector, a stratified storage tank (either vertical or horizontal cylinder), and a check valve to prevent reverse flow, which employs water as the working fluid. In fact TYPE 45a is used in which the head and flow rate are calculated internally by the program. Flow in the loop is assumed to be steady state. The system is analysed by dividing the thermosiphon loop into a number of segments normal to the flow direction and applying Bernoulli’s equation for incompressible flow to each segment. The flow rate is obtained by numerical solution of the resulting set of equations. The stratification in the tank is modelled using TYPE 38 algebraic component, which is embodied in TYPE 45. The number of segments (nodes) in this model is not fixed, but depends on many factors, i.e., the simulation time step, the size of the collector, load flow rates, heat losses and auxiliary input (Fanney and Klein, 1983). In this model the simulation starts with a certain number of segments. As the hot water leaves the top of the collector and enters the storage tank from a certain point at the top, it mixes with water at this level if their temperatures are within 0.5 °C. If its temperature is lower than that at the top by more than 0.5 °C it flows down and mixes with water of a segment where the temperature is within 0.5 °C of it. In the case when the temperature of water entering the storage tank is higher than that at the top of the storage tank by more than 0.5 °C, a new segment is created at the top, increasing the number of segments by one. When hot water is drawn to the load, the same case applies for cold water from the mains entering the tank at the bottom, this water mixes with that at the bottom if their temperatures are within 0.5 °C of each other, otherwise a new segment is created. The system presented in this paper is simulated with TRNSYS using typical meteorological year (TMY) data for Nicosia, Cyprus. TMY is defined as a year, which sums up all the climatic information characterizing a period as long as the mean life of the system. The selection of typical weather conditions for a given location is very crucial in computer simulations for performance predictions and has led various investigators either to run long periods of observational data or to select a particular year, which appears to be typical from several years of data. The TMY for Nicosia, Cyprus, was generated from hourly measurements, of solar irradiance (global and diffuse on horizontal surface), ambient temperature, wind speed and
43
direction and humidity ratio, for a seven-year period, from 1986 to 1992 using the Filkenstein–Schafer statistical method (Kalogirou, 2003b). The measurements were recorded by the Cyprus Meteorological Service at the Athalassa region, an area at the suburbs of the town of Nicosia. The TMY is considered as a representative year for the Cypriot environment. 4.1. Economic analysis For this analysis, a life cycle analysis, in which the various costs are estimated annually, is considered. A life cycle analysis is performed in order to obtain the total cost (or life cycle cost) and the life cycle savings (LCS) of the system. This kind of economic analysis can be performed either within the TRNSYS environment or in a spreadsheet program (Kalogirou, 1996). The TRNSYS application is preferred here, as its output is given directly and avoids subsequent transfer of the thermal performance results to a spreadsheet. In general, the present worth (or discounted cost) of an investment or cost C at the end of year N, at a discount rate of d and interest rate of i is obtained by PWN ¼
Cð1 þ iÞN 1 ð1 þ dÞN
ð7Þ
In this method, the various costs and savings are estimated annually. From the addition of fuel savings incurred because of the use of the system and the tax savings, the mortgage, maintenance and parasitic costs are subtracted and thus the annual solar savings of the system are estimated which are converted into present worth values of the system. These are added up to obtain the life cycle savings according to the equation PWLCS ¼
N X Solar Savings N ð1 þ dÞ N ¼1
ð8Þ
The fuel savings are obtained by subtracting the annual cost of the conventional fuel used for the auxiliary energy from the fuel needs of a fuel only system. The integrated cost of the auxiliary energy use for the first year, i.e., solar backup, is given by the formula Z t C FA Qaux dt ð9Þ C aux ¼ 0
The integrated cost of the total load for the first year, i.e., cost of conventional fuel without solar, is Z t C FL Qload dt ð10Þ C load ¼ 0
where CFA and CFL are the cost rates for auxiliary energy and conventional fuel, respectively. In case that the same fuel is used for both CFA = CFL. The investment cost of the solar system in this case is estimated from the following equation:
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S. Kalogirou / Solar Energy 83 (2009) 39–48
Cs ¼ Cf þ CaA
ð11Þ
Referring to the curve of hot water load (Qload), there is a decrease of hot water load demand during the summer months. This is attributed to the fact that during the summer months the total incidence solar radiation is higher which results to higher temperatures in the cold water storage tank. Consequently, the hot water demand from the hot water storage tank during these months is reduced. The variation of the annual solar contribution is also shown in Fig. 3. The solar fraction, f, is defined as the ratio of the useful solar energy supplied to the system to the energy needed to heat the water if no solar energy is used. In other words, f is a measure of the fractional energy savings relative to that used for a conventional system. It can be calculated from the following relationship:
where Cf is the collector area independent and Ca the collector area dependent costs. The values used here are Cf = 153 € and Ca = 128 €/m2. The maintenance cost (Cm), must also be considered. This is estimated to be 1% of the initial investment and is assumed to increase at a rate of 1% per year of the system operation. The total annual cost is given by C ¼ Cs þ Cm
ð12Þ
4.2. Results of the thermal and economic analysis Fig. 3 illustrates the monthly energy flows, which include the total radiation incident on the collector (Qins), the useful energy supplied from the collectors (Qu), the hot water energy requirements (Qload), the auxiliary energy demand (Qaux) and the heat losses from the storage tank (Qenv). As it can be seen from the graph of total radiation incident on collector (Qins), the maximum value occurs in the month of August (1.88 GJ). The useful energy supplied from the collectors (Qu) is maximized in the month of April (0.62 GJ). It can be also seen from Fig. 3 that there is a reduction in the incident solar radiation and consequently the useful energy collected during the month of May. This is a characteristic of the climatic conditions of Nicosia and is due to the development of clouds as a result of excessive heating of the ground and thus excessive convection, especially in the afternoon hours. The annual value of the useful energy supplied by solar energy (Qu) is equal to 6480 MJ. From the curve of the energy lost from storage tank (Qenv) it can be seen that during summer months the energy lost from storage to surroundings is maximized. This is due to the fact that at these months the temperature in the storage tank is higher and consequently more energy is lost.
f ¼
Qload Qaux Qload
ð13Þ
Fig. 3 implies that the solar fraction is lower in the winter months and higher, reaching 100%, in the summer months. The annual solar fraction is determined to be 79%. It should be noted that in domestic hot water systems, by adjusting slightly the consumption profile, contributions of 100% could be obtained in the months May–October, which is what actually happens in practice. The program however considers a standard consumption throughout all months that is why values slightly below 100% are given. The economic scenario used in this project is that all the cost of the solar system is paid from the beginning (i.e., no credit payments are assumed). The thermal performance degradation of the system is assumed to be 1% per year, the period of economic analysis is taken as 20 years (average life of locally produced systems), whereas all the other percentage figures (inflation rates and market discount rate) are mean values of the previous decade (Statistical abstracts, 2001). Electricity at a price of 0.153 €/kW h
2.00
1.00
1.80
0.90 0.80
ff Qins Qins Qu Qu Qload Qload Qaux Qaux Qenv Qenv
1.40 1.20 1.00 0.80
0.70 0.60 0.50 0.40
0.60
0.30
0.40
0.20
0.20
0.10
0.00
0.00 JAN
FEB
MAR
APR
MAY
JUN
JUL
AUG
SEP
OCT
NOV
Months
Fig. 3. Energy flows of the thermosiphon solar water heater.
DEC
YR
Solar Fraction
Energy Flows (GJ)
1.60
S. Kalogirou / Solar Energy 83 (2009) 39–48 Table 2 Results of economic analysis Output parameter
Electricity backup
Diesel backup
Initial cost of the system (€) Resale or salvage value (€) Rate of return of solar investment (%) Years until undiscounted fuel savings = investment Undiscounted cumulative net cash flow (€) Present worth of total costs with solar (€) Present worth of total costs without solar (€) Annualized total cost with solar (€/GJ) Annualized total cost without solar (€/GJ) Present worth of cumulative cash flow (€) Fuel price considered in the analysis
498 98 39.9 2.7
498 98 23.4 4.5
4616 1209 3450
2417 929 1985
18.7 54.4 2240 0.153 €/kW h
14.5 31.5 1056 0.76 €/l
and diesel at a price of 0.76 €/l are assumed to be used for auxiliary. Table 2 gives a summary of economic figures as calculated by TRNSYS. The results of the economic analysis shown in Table 2 were obtained by using the above (current) fuel and electricity rates, a 20-year period a market discount rate of 6% and interest rate of 8%. No subsidies were considered. As can be seen the solar system gives much lower specific energy cost than the conventional system for both types of auxiliary energy considered. The payback time is very low and is equal to two years and eight months for electricity backup and four years and six months for diesel backup. The life cycle savings represent the money that the owner will save by installing the solar system instead of buying electricity/fuel to satisfy his hot water needs and is equal to 2240 € for electricity backup and 1056 € for diesel backup. The difference in payback time and life cycle savings is due to the fact that in the case of diesel a fuel of lower price is replaced by the solar system. 5. Environmental benefits of thermosiphon solar water heating systems To investigate the environmental benefit of utilizing solar energy instead of conventional sources of energy, the different emissions resulting from the solar system operation are estimated and compared to those of a conventional electricity/fuel system. These are obtained by correlations derived by using program Polysun (2000) given in (Kalogirou, 2004b). The emissions reported are those which are responsible for the most important environmental problems as outlined in the previous sections. The environmental interventions are expressed in physical units of the emitted substances per year. The quantities of the emissions depend on the solar collector size and the required auxiliary energy and are compared to a non-solar system which is using conventional electricity/fuel. The environmental analysis of the system which includes the different pollutants as calculated by the program is tabulated in Tables 3 and 4 for electricity and diesel backup, respectively. In the tables the eight most
45
Table 3 Environmental impact of the thermosiphon solar water heater with electricity backup Emissions
Units
Conventional
Carbon dioxide (CO2) Carbon monoxide (CO) Nitrogen oxides (NOx) Nitrous oxide (N2O) Methane (CH4) Hydrocarbons Sulphur dioxide (SO2) Dust Savings in GHG
tons/ year g/year
374.6
109.7
70.7
g/year
56.3
16.3
71.1
g/year g/year g/year g/year
6.3 9.3 37.7 562.7
2.1 2.7 11.0 164.5
66.7 71.0 70.8 70.8
g/year %
188.1 –
54.8 –
70.9 70.3
1.546
Solar system 0.449
Savings (%) 70.1
Table 4 Environmental impact of the thermosiphon solar water heater with diesel backup Emissions
Units
Conventional
Solar system
Savings (%)
Carbon dioxide (CO2) Carbon monoxide (CO) Nitrogen oxides (NOx) Nitrous oxide (N2O) Methane (CH4) Hydrocarbons Sulphur dioxide (SO2) Dust Savings in GHG
tons/ year g/year
0.889
0.293
67.1
1688
581.3
65.6
g/year
1636
544.8
66.7
g/year g/year g/year g/year
6.1 13.6 52.7 651.4
1.2 3.3 12.9 169.9
80.0 75.7 75.5 73.9
g/year %
180.0 –
73.5 –
59.2 70.5
important greenhouse gasses are considered. For the case of electricity backup (Table 3), Polysun considers a mixture of European power stations (coal-based, nuclear, hydroelectric, etc.) in order to estimate the emissions of the conventional system (Polysun, 2000). As can be seen in both cases by using solar energy instead of conventional fuel very large amount of pollutants are avoided. The amount of emissions depends on the type of fuel used as auxiliary. The percentage saving obtained in the cases where electricity or diesel backup is used is about 70%. It should be noted however that the quantities of emissions in the various substances emitted are completely different and the proximity of the total percentage numbers obtained is due to the generation efficiency of each system. Electrical energy is produced at a maximum efficiency of about 35% whereas in the case of diesel backup a boiler efficiency of 85% is considered. 6. Pollution created from thermosiphon solar water heating systems The negative environmental impact of any solar energy system includes land displacement, and possible air and
46
S. Kalogirou / Solar Energy 83 (2009) 39–48
water pollution resulting from the manufacture, normal maintenance operations and demolition of the systems. However, land use is not a problem when collectors are mounted on the roof of a building, maintenance required is minimal and pollution caused by demolition is not greater than the pollution caused from demolishing a conventional system of the same capacity, and the great majority of the solar system components can be recycled, therefore not disposed to the environment. The pollution created for the manufacture of the solar collectors is estimated by calculating the embodied energy invested in the manufacture and assembly of the collectors and estimating the pollution produced by this energy. Initially, the embodied energy of one solar collector panel, 1.35 m2 in area is determined. This is the same collector considered in the performance analysis of the system. The analysis is based on the primary and intermediate embodied energy of the components and materials as illustrated in Fig. 4. In the present analysis no allowance is made for the unit packing, transportation and maintenance as these have insignificant contribution compared to the total. The total embodied energy required to produce a complete flat-plate collector is calculated using primary and intermediate production stages. The primary stage is established from an assessment of the various materials used and their corresponding mass. Using the embodied energy index (MJ/kg) defined by Alcorn (1995) the material embodied energy content within the unit is determined. Table 5 summarizes the unit materials used and lists their corresponding mass and embodied energy content. The total embodied energy content for the production of one flat-plate collector panel is calculated at 2663 MJ. This comprise the primary embodied energy of materials and the intermediate embodied energy, i.e., the amount of
PRIMARY PRODUCTION
Table 5 Embodied energy content of one flat-plate collector 1.35 m2 in area Description
Mass (kg)
Embodied energy index (MJ/kg)
Embodied energy content (MJ)
1.6 0.85 0.05 m insulation 1.6 0.85 0.005 m glass 1.8 m, 22 mm copper pipe 16 m, 15 mm copper pipe 2 1.05 0.005 m galvanized steel sheet 5 m rubber sealant Black paint Casing paint 20 No. screws 169 0.85 0.003 m copper absorber
4.3
117
503.1
9.5
15.9
151.1
2.16
70.6
152.5
9.9
70.6
698.8
8.2
34.8
285.4
0.5 0.3 0.9 0.00125 3.6
110 44 44 34.8 70.6
55 13.2 39.6 Ignored 254.2
Total Add 10% for contingencies Unit manufacture using a net to gross value of conversion rate of 27%
2153 215 295
Grant total
2663
energy used in the production and assembly of the component parts during the construction stage and was determined through a stage-by-stage appraisal of the power sources used. Inherent within this intermediate stage is the fabrication of purchased components like screws, glass and insulation. An analysis of the embodied energy content of a complete thermosiphon solar water heating system is shown in Table 6. It should be noted that only the extra compo-
INTERMEDIATE PRODUCTION
Paint, sealant, coatings Primary raw materials extraction and production
Glass Copper pipes
Absorber
Copper Sheet Insulation Galvanized sheet
Casing
Packing Transportation Installation Maintenance Demolition Disposal/recycling
Fig. 4. Factors considered in the calculation of embodied energy of a flat-plate collector.
S. Kalogirou / Solar Energy 83 (2009) 39–48 Table 6 Embodied energy content for the construction and installation of the complete thermosiphon solar water heating system Description 2 No. solar panels 4 m, 22 mm copper pipe 4 m, pipe insulation Steel frame
Mass (kg) – 3.8 1 30
Embodied energy index (MJ/kg) – 70.6 120 34.8
Embodied energy content (MJ)
Table 7 Pollution created for the construction and installation of the thermosiphon solar water heating system and payback for the two types of backup fuels considered Emission
Pollution created from solar system embodied energy
Savings and payback of solar system Electricity
Diesel
Carbon dioxide (CO2) Carbon monoxide (CO) Nitrogen oxides (NOx) Nitrous oxide (N2O) Methane (CH4) Hydrocarbons
1.9 tons
1.097 (1.7)
0.596 (3.2)
460.1 g
264.9 (1.7)
1106.7 (0.4)
69.2 g
40 (1.7)
1091.2 (0.06)
6.3 g
4.2 (1.5)
11.4 g
6.6 (1.7)
46.3 g
26.7 (1.7)
691.2 g
398.2 (1.7)
4.9 (1.3) 10.3 (1.1) 39.8 (1.2) 481.5 (1.4)
231.1 g
133.3 (1.7)
106.5 (2.2)
5326 268.3 120 1044
Total Installation
6758.3 187.7
Grant total
6946
nents of the solar system are considered in this analysis as the other components are standard and are also present in conventional systems. Here, the objective is to compare the pollution created for the manufacture and installation of the solar system against its benefits due to the lower emissions realized during the operation of the system. As can be seen the total embodied energy for the complete system is 6946 MJ. For the life cycle assessment of the system considered the useful energy supplied by solar energy per year, shown in Fig. 3 (6480 MJ) is compared with the total embodied energy of the system shown in Table 6. As can be seen the total energy used in the manufacture and installation of the system is recouped in about 13 months which is considered as very satisfactory. The emissions created from total embodied energy are presented in Table 7. Additionally, these emissions are compared with the emissions produced from the auxiliary energy, according to the type of fuel used in the various cases investigated, in order to estimate the payback period for each pollutant. In all cases the emissions are estimated by considering that all embodied energy was produced from electricity. This is not quite correct but electricity is chosen, as is the most polluting fuel, therefore it gives the worst possible results. As can be seen from Table 7, the payback periods for the cases investigated vary from a few months to 3.2 years according to the fuel and the particular pollutant considered. The size of thermosiphon solar water heater considered is the usual type encountered in Cyprus. Cyprus began manufacturing solar water heaters in the early sixties. Today more than 93% of all houses have solar water heating systems installed and operating. The total number of systems is equal to 190,000 units. In fact the number of units in operation today corresponds to one heater for every 3.7 people in the island, which is a world record (Kalogirou, 2003a). If we consider that all systems are of the same size as the one investigated here and that 50% of the systems are using electricity as backup and 50% diesel then the amount of pollutants avoided per year are as shown in Table 8. As can be seen considerable environmen-
47
Sulphur dioxide (SO2) Dust
Note: (1) Number in parenthesis represent payback time in years. (2) The units of savings are in g/year except carbon dioxide which is tons/year.
Table 8 Annual environmental pollution reduction because of the use of thermosiphon systems in Cyprus Emission
Savings
Carbon dioxide (CO2) Carbon monoxide (CO) Nitrogen oxides (NOx) Nitrous oxide (N2O) Methane (CH4) Hydrocarbons Sulphur dioxide (SO2) Dust
160,835 tons 130.3 tons 107.5 tons 864.5 kg 1605.5 kg 6317.5 kg 83.6 tons 22.8 tons
Notes: 1. Total number of units considered = 190,000. 2. Savings estimated by considering that 50% of the systems are using electricity and 50% diesel as backup.
tal pollution reduction occurs each year just for water heating. Moreover, the cost of damage avoided by some of the pollutants is investigated with respect to damages to crops, materials, mortality (refers to premature deaths) and morbidity (refers to illness). The results of this analysis are shown in Tables 9 and 10. As can be seen 36 € are avoided per year when the system is using electricity as auxiliary and 22.4 € when diesel is used for each thermosiphon solar water heating system. Therefore for a more correct analysis of solar systems the damage cost avoided, shown in Table 10, should be added to the annual fuel savings which will reduce the payback time even more, thus there is a further
48
S. Kalogirou / Solar Energy 83 (2009) 39–48
Table 9 Typical damage costs per kilogram of pollution emitted by power plants in Europe (Rabl and Spardaro, 2001) Pollutant
Impact
Cost (€/kg)
SO2 CO (primary) CO2
Crops, materials, mortality and morbidity Morbidity Global warming
10.55 0.002 0.029
Table 10 Damage cost avoided per year from some of the pollutants for thermosiphon solar water heating systems Pollutant
CO2 CO SO2
Amount saved (kg)
Damage cost avoided (€)
Electricity
Diesel
Electricity
Diesel
1097 0.265 0.398
596 1.107 0.482
31.8 0 4.2
17.3 0 5.1
36
22.4
Total
increase in the economic viability of the systems. It is believed by the author that such type of analysis must always be considered in feasibility studies of solar systems. 7. Conclusions In the present study, the potential benefits that solar systems offer are discussed in detail. From the analysis presented in this paper it can be concluded that the environmental impact of any energy system is an important factor and solar systems have the potential to reduce environmental pollution. Additionally, in this study the environmental protection offered by the most widely used renewable energy system, i.e., the thermosiphon solar water heating system is presented. The results show that by using solar energy considerable amounts of greenhouse polluting gasses are saved. For the domestic solar heating system considered here, with electricity or diesel backup the saving, compared to a conventional system, is about 70%. Additionally, the system investigated give positive and very promising performance and financial characteristics. The annual solar contribution is 79% whereas the payback time of the system is 2.7 year and the life cycle savings are 2240 € for electricity backup and 4.5 years and 1056 € for diesel backup, respectively. With respect to life cycle assessment of the systems, the energy spent for the manufacture and installation of the solar systems is recouped in about 13 months, whereas the payback time with respect to emissions produced from the embodied energy required for the manufacture and installation of the systems varies from a few months to 3.2 years according to the fuel and the particular pollutant considered. Moreover, the cost of damage avoided by some of the pollutants is investigated with
respect to damages to crops, materials, mortality and morbidity and found that 36 € are avoided per year when the system is using electricity as auxiliary and 22.4 € when using diesel. It can therefore be concluded that solar energy systems are efficient, cost effective and friendlier to the environment. The reduction of greenhouse gasses pollution is the main advantage of utilizing solar energy. Therefore, solar energy systems should be employed whenever possible in order to achieve a sustainable future. References Alcorn, J., 1995. Embodied energy coefficients of building materials. Centre for Building Performance Research, Victoria University of Wellington, New Zealand. Argiriou, A., Klitsikas, C., Balaras, C., Asimakopoulos, D., 1997. Active solar space heating of residential buildings in northern Hellas – a case study. Energy and Buildings 26 (2), 215–221. Diakoulaki, D., Zervos, A., Sarafidis, J., Mirasgedis, S., 2001. Cost benefit analysis for solar water heating systems. Energy Conversion and Management 42 (14), 1727–1739. Fanney, A.H., Klein, S.A., 1983. Performance of solar domestic hot water systems at the National Bureau of Standards – measurements and predictions. ASME Solar Energy Engineering 105, 311–321. Hasan, A., 2000. Optimisation of collector area in solar water heating systems. International Journal of Solar Energy 21 (1), 19–27. Kalogirou, S., 1996. Economic analysis of solar energy systems using spreadsheets. In: Proceedings of the World Renewable Energy Congress IV, Denver, CO, USA. vol. 2. pp. 1303–1307. Kalogirou, S., 2003a. The energy subsidisation policies of Cyprus and their effect on renewable energy systems economics. Renewable Energy 28 (11), 1711–1728. Kalogirou, S., 2003b. Generation of typical meteorological year (TMY-2) for Nicosia, Cyprus. Renewable Energy 28 (15), 2317–2334. Kalogirou, S., 2004a. Solar thermal collectors and applications. Progress in Energy and Combustion Science 30 (3), 231–295. Kalogirou, S., 2004b. Environmental benefits of domestic solar energy systems. Energy Conversion and Management 45 (18–19), 3075–3092. Keyanpour-Rad, M., Haghgou, H.R., Bahar, F., Afshari, E., 2000. Feasibility study of the application of solar heating systems in Iran. Renewable Energy 20 (3), 333–345. Mirasgedis, S., Diakoulaki, D., Assimacopoulos, D., 1996. Solar energy and the abatement of atmospheric emissions. Renewable Energy 7 (4), 329–338. Rabl, A., Spardaro, J.V., 2001. The cost of pollution and the benefit of solar energy. In: Gordon, J.M. (Ed.), Solar Energy: The State of the Art ISES. James and James Ltd., London, pp. 437–475, (Chapter 8). Polysun, 2000. User’s manual for Polysun 3.3. SPF, Switzerland. Statistical abstract, 2001. Statistical service. Republic of Cyprus, Report No. 44. Taborianski, V.M., Prado, R.T.A., 2004. Comparative evaluation of the contribution of residential water heating systems to the variation of greenhouse gases stock in the atmosphere. Building and Environment 39 (6), 645–652. TRNSYS, 2005. Version 16 program manual. Solar Energy Laboratory, University of Wisconsin, Madison, USA. Tsillingirides, G., Martinopoulos, G., Kyriakis, N., 2004. Life cycle environmental impact of a thermosiphon domestic solar hot water system in comparison with electrical and gas heating. Renewable Energy 29 (8), 1277–1288.
Solar Energy 83 (2009) 39–48 www.elsevier.com/locate/solener
Thermal performance, economic and environmental life cycle analysis of thermosiphon solar water heaters Soteris Kalogirou * Cyprus University of Technology, Department of Mechanical Engineering and Materials Sciences and Engineering, P.O. Box 50329, Lemesos 3603, Cyprus Received 29 January 2007; received in revised form 5 November 2007; accepted 23 June 2008 Available online 18 July 2008 Communicated by: Associate Editor Dr. D. Yogi Goswami
Abstract In this paper, the environmental benefits or renewable energy systems are initially presented followed by a study of the thermal performance, economics and environmental protection offered by thermosiphon solar water heating systems. The system investigated is of the domestic size, suitable to satisfy most of the hot water needs of a family of four persons. The results presented in this paper show that considerable percentage of the hot water needs of the family are covered with solar energy. This is expressed as the solar contribution and its annual value is 79%. Additionally, the system investigated give positive and very promising financial characteristics with payback time of 2.7 years and life cycle savings of 2240 € with electricity backup and payback time of 4.5 years and life cycle savings of 1056 € with diesel backup. From the results it can also be shown that by using solar energy considerable amounts of greenhouse polluting gasses are avoided. The saving, compared to a conventional system, is about 70% for electricity or diesel backup. With respect to life cycle assessment of the systems, the energy spent for the manufacture and installation of the solar systems is recouped in about 13 months, whereas the payback time with respect to emissions produced from the embodied energy required for the manufacture and installation of the systems varies from a few months to 3.2 years according to the fuel and the particular pollutant considered. It can therefore be concluded that thermosiphon solar water hearting systems offer significant protection to the environment and should be employed whenever possible in order to achieve a sustainable future. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Thermosiphon; Solar water heating; Thermal performance; Economic analysis; Embodied energy; Environmental life cycle analysis
1. Introduction All nations of the world depend on fossil fuels for their energy needs. However, the obligation to reduce CO2 and other gaseous emissions, in order to be in conformity with the Kyoto agreement is the reason behind which countries turn to non-polluting renewable energy sources. In developed countries, energy consumption in the building sector represents a major part of the total energy budget. In the European Union this is approximately equal *
Tel. +357 2500 2621; fax: +357 2500 2769. E-mail addresses: [email protected], cut.ac.cy
soteris.kalogirou@
0038-092X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2008.06.005
to 40% of the total energy consumption (Argiriou et al., 1997). A considerable percentage of this amount is spent for hot water production. One way to reduce this amount of energy is to employ solar energy. In literature, there are numerous studies on the environmental life cycle analysis of a variety of thermal systems. Some of them deal with solar water heating systems (Mirasgedis et al., 1996; Taborianski and Prado, 2004; Tsillingirides et al., 2004). Particularly, the latter study examined the life cycle environmental impact of a thermosiphon domestic solar hot water system in comparison with electrical and gas heating in Greece. In some other studies only the economic life cycle savings are reported (Hasan, 2000; Keyanpour-Rad et al., 2000).
40
S. Kalogirou / Solar Energy 83 (2009) 39–48
Nomenclature A b0 C Ca Caux c0 c1 c2 Cf CFA CFL CFC Cload Cm Cs d f FR Gt i kas
collector area (m2) constant of incident angle modifier equation investment cost (€) collector area dependent cost (€/m2) cost of auxiliary energy (€) intercept efficiency negative of the first-order coefficient of the efficiency (W/m2 °C) negative of the second-order coefficient of the efficiency (W/m2 °C2) collector area independent cost (€) cost rate of auxiliary energy (€/kJ) cost rate of conventional fuel (€/kJ) chlorofluorocarbons cost of fuel to cover load (€) maintenance cost (€) collector investment cost (€) market discount rate (%) solar contribution heat removal factor total global solar radiation (W/m2) interest rate (%) incidence angle modifier
In this paper, initially the environmental benefits of renewable energy systems are shortly discussed, followed by the analysis of the thermal performance, economics and environmental benefits resulting from the use of thermosiphon solar water heating systems. Additionally, the amount of pollution saved because of the use of solar energy against the pollution caused for the manufacture of the systems is examined. 2. Environmental benefits of renewable energy technologies Renewable energy technologies produce marketable energy by converting natural phenomena into useful forms of energy. These technologies use the sun’s energy and its direct and indirect effects on the earth as the resources from which energy is produced. These resources have massive energy potential, however, they are generally diffused and not fully accessible, most of them are intermittent, and have distinct regional variabilities. These characteristics give rise to difficult, but solvable, technical and economical challenges. Today, significant progress is made by improving the collection and conversion efficiencies, lowering the initial and maintenance costs and increasing the reliability and applicability of renewable energy systems (Kalogirou, 2004b). Two potential solutions to the current environmental problems associated with the harmful pollutant emissions from the burning of fossil fuels are renewable energy and
LCS N g PW Qins Qenv Qu Qaux Qload Qloss Ta Ti TMY UL UV VOC
life cycle savings (€) number of years collector efficiency present worth (€) total radiation incident on the collector (kJ) heat losses from storage tank (kJ) useful energy supplied form solar collectors (kJ) auxiliary energy (kJ) energy required to cover load (kJ) the annual energy lost from the storage tank and pipes (kJ) ambient temperature (°C) inlet temperature to the collector (°C) typical meteorological year heat loss coefficient (W/m2 °C) ultraviolet volatile organic compound
Greek symbols DT temperature difference [=Ti Ta] (°C) h incidence angle (degrees) sa transmittance–absorptance product
energy conservation technologies. Many countries consider today solar, wind and other renewable energy technologies as the key to a clean energy future. Renewable energy systems can have a beneficial impact on the environmental, economic, and political issues of the world (Kalogirou, 2004b). The benefits arising from the installation and operation of renewable energy systems can be distinguished into three categories: energy saving, generation of new working posts and the decrease of environmental pollution (Diakoulaki et al., 2001). The most important benefit of renewable energy systems is the decrease of environmental pollution. This is achieved by the reduction of air emissions due to the substitution of electricity and conventional fuels. The most important effects of air pollutants on the human and natural environment are their impact on the public health, agriculture, buildings and historical monuments, as well as on forests and ecosystems (Diakoulaki et al., 2001). It is relatively simple to measure the financial impact of these effects when they affect tradable goods such as the agricultural crops; but this is much more complicated when it comes to non-tradable goods like human health and ecosystems. This paper deals with thermosiphon solar water heating systems, which are very popular systems, used extensively in many countries with good sunshine potential such as the Mediterranean countries. A domestic size system is considered, which is analysed with respect to its
S. Kalogirou / Solar Energy 83 (2009) 39–48
energy performance, impact.
economics
and
environmental
3. Thermosiphon solar water heating systems Thermosiphon, or natural circulation, solar water heating systems (also called passive systems) are the simplest and most widely used solar energy collection and utilization devices (Fig. 1). They are intended to supply hot water for domestic use, and are based on natural circulation or thermosiphon principle. They supply hot water at a temperature of about 60 °C and consist of a collector, storage tank, and connecting pipes. A schematic diagram of the thermosiphon systems is shown in Fig. 1a. These systems heat potable water or a heat transfer fluid and use natural convection to transport it from the collector to storage. Thermosiphoning occurs when the water in the collector expands becoming less dense as heat is added by solar energy and rises through the collector header into the top of the storage tank. There it is replaced by the cooler water that has sunk to the bottom of the tank from which it flows down the collector. Circulation continuous as long as the sun is shining. Since the driving force is only a small density difference between the hot and cold water, larger than normal pipe sizes must be used to minimize pipe friction. In the storage tank, hot water accumulates near the top when water is heated during the day by solar radiation. To take into account periods of low solar radiation levels, storage tanks are normally sized to hold about two days’ supply of hot water. It should be noted that the water flowing through the collectors is potable water that goes to the user and any quantity of hot water used is replaced through the freshwater inlet (from the cold water storage tank or mains supply) which enters the storage tank near the bottom so as not to break the stratification.
41
Connecting lines must be well insulated to prevent heat losses and sloped to prevent formation of air pockets which would stop circulation. At night, or whenever the collector is cooler than the water in the tank the direction of the thermosiphon flow will reverse, thus cooling the stored water, unless the top of the collector is placed well below (about 30 cm) the bottom of the storage tank (Kalogirou, 2004a). The size of a thermosiphon solar system depends on the prevailing weather conditions and the hot water requirements. The collector area is determined primarily by the daily hot water demand, which varies from place to place depending on local customs and lifestyles and is normally about 30 l/person/day. A typical unit operating in a good environment (Mediterranean area) usually consists of two flat-plate solar collectors having an absorber area between 2.5 and 4 m2, and a storage tank with capacity between 150 and 180 l. An auxiliary electric immersion heater and/or a heat exchanger, for central heating assisted hot water production, are used in winter during periods of low solar insolation. Such a system covers about 80% of the hot water requirements of a four-person family. The flat-plate collector is generally fixed permanently in position, and therefore the tilt of the collector is determined primarily by consideration of the predominant season of hot water use. For year-round use, the collector tilt is kept equal to the latitude of the location plus 5°. In the northern hemisphere, the collector faces direct south (azimuth angle = 0°), although a shift of a few degrees towards east or west does not greatly influence its performance (Kalogirou, 2004a). The daily overall system efficiency of a domestic solar hot water system is about 30–40%, and the temperature difference between the collector outlet and inlet is about 10 °C. The storage tank is placed horizontally or vertically. Though the shallow depth of the horizontal tanks degrades stratification, for horizontal tanks with
Mixing device Storage tank
Hot water outlet
Auxiliary Cold water inlet
Collector
(a) Schematic diagram
(b) Photograph
Fig. 1. Schematic diagram and a photograph of a thermosiphon solar water heater.
S. Kalogirou / Solar Energy 83 (2009) 39–48
diameters greater than 500 mm there is only a small performance loss in comparison with vertical tanks. The main disadvantage of thermosiphon systems is the fact that they are comparatively tall units, which makes them not very attractive aesthetically. Usually, a cold water storage tank is installed on top of the solar collector, supplying both the hot water cylinder and the cold water needs of the house, thus making the collector unit taller and even less attractive (see Fig. 1b). Additionally, extremely hard or acidic water can cause scale deposits that may clog or corrode the absorber fluid passages. For direct systems, pressure-reducing valves are required when the city water is used directly and pressure is greater than the working pressure of the collectors. The unit considered in this work employs flat-plate collectors which are by far the most used type of collector. The instantaneous efficiency of the collector considered is given by the following equation obtained by testing the collector at the Applied Energy Centre of the Ministry of Commerce and Industry, Nicosia, Cyprus: 2 DT DT 0:06 ð1Þ n ¼ 0:792 6:65 Gt Gt where DT is temperature difference between the collector inlet (Ti) and ambient (Ta) temperatures, i.e., DT = (Ti Ta), and Gt is the global solar radiation. The storage tank is well insulated to reduce thermal loses to the environment and is equipped with heat exchangers for heating the water with auxiliary energy. The auxiliary can be either electricity or diesel. In the case where diesel is considered this is used in a central heating boiler, which supplies the energy for the heating needs of a house and is not used only as the solar system backup. What is of interest to note is that if the temperature of the water in the storage tank is more than the desired temperature this is mixed with the make-up water to obtain the required temperature. This is done at the tap by the user (through the mixer) but in the simulation, it is done with the mixing devise shown in Fig. 1a. The specifications of the various components of the solar system are shown in Table 1. With regard to the thermal load, although the hot water demand is subject to a high degree of variation from day to day and from consumer to consumer it is impractical to use anything but a repetitive load profile. This is not quite correct during the summer period, where the consumption Table 1 Specifications of the thermosiphon solar water system considered Parameter
Domestic hot water system 2
Collector area (m ) Collector slope (°) Storage capacity (l) Auxiliary capacity (kW) Heat exchanger Heat exchanger area (m2) Hot water demand (l)
2.7 (2 panels) 40 150 3 Internal 3.6 120 (4 persons)
Hot water consumption (l)
42
15 10 5 0 0
2
4
6
8
10 12
14
16
18
20
22
24
Hours
Fig. 2. Hot water daily consumption profile.
pattern is somewhat higher. However, during this period, the temperature requirement for hot water is not as high as during winter. Consequently, the total thermal energy requirement is reasonably constant throughout the year. For the present simulation, the hot water consumption profile illustrated in Fig. 2 is employed, which assumes a daily hot water consumption of 120 l at 50 °C for a family of four (30 l/person). Traditional hot water systems comprise a hot water cylinder powered either by electricity or by diesel oil through the central heating boiler. Therefore, the extra equipments required for the solar system are the solar collectors and the piping to connect the collectors with the storage tank. 4. Thermal and economic analysis of the solar system The system is modelled with the well-known TRNSYS simulation program. TRNSYS is employing the standard second-order collector performance equation to model the collector, given by (TRNSYS, 2005) n ¼ c0 K h c1
ðT i T a Þ ðT i T a Þ c2 Gt Gt
2
where Kh is the incidence angle modifier given by 1 1 K h ¼ 1 b0 cosðhÞ
ð2Þ
ð3Þ
The values of c0, c1, c2 and b0 are obtained by experimental testing of collectors in accredited laboratories. For the present application the factors shown in Eq. (1) are used, for c0, c1 and c2, whereas b0 is equal to 0.1, also determined from testing the system in the test centre described above. The useful energy extracted from the collectors is given by Qu ¼ F R A½Gt ðsaÞK h U L ðT i T a Þ
ð4Þ
The total useful energy for the whole year is obtained from Qu;a ¼
365 X 24 X
Qu
ð5Þ
d¼1 h¼1
and the auxiliary energy required, Qaux is Qaux ¼ Qload ½Qu;a Qloss
ð6Þ
where Qload is the energy required by the load and Qloss is the energy lost from the storage tank and pipes.
S. Kalogirou / Solar Energy 83 (2009) 39–48
As can be seen from the above equations the energy obtained from the solar collector field depends on the collector area (A), collector slope (affects cosh), flow rate (affects FR) and the storage tank size (affects Ti). The collector inlet temperature depends also on the load pattern, make-up water temperature and the losses from the storage tank and pipes. The storage tank losses depend on the temperature of the stored water, i.e., it depends on the energy collected and storage tank size. The main component that models the thermosiphon system is TYPE 45. This component models a system consisting from a flat-plate solar collector, a stratified storage tank (either vertical or horizontal cylinder), and a check valve to prevent reverse flow, which employs water as the working fluid. In fact TYPE 45a is used in which the head and flow rate are calculated internally by the program. Flow in the loop is assumed to be steady state. The system is analysed by dividing the thermosiphon loop into a number of segments normal to the flow direction and applying Bernoulli’s equation for incompressible flow to each segment. The flow rate is obtained by numerical solution of the resulting set of equations. The stratification in the tank is modelled using TYPE 38 algebraic component, which is embodied in TYPE 45. The number of segments (nodes) in this model is not fixed, but depends on many factors, i.e., the simulation time step, the size of the collector, load flow rates, heat losses and auxiliary input (Fanney and Klein, 1983). In this model the simulation starts with a certain number of segments. As the hot water leaves the top of the collector and enters the storage tank from a certain point at the top, it mixes with water at this level if their temperatures are within 0.5 °C. If its temperature is lower than that at the top by more than 0.5 °C it flows down and mixes with water of a segment where the temperature is within 0.5 °C of it. In the case when the temperature of water entering the storage tank is higher than that at the top of the storage tank by more than 0.5 °C, a new segment is created at the top, increasing the number of segments by one. When hot water is drawn to the load, the same case applies for cold water from the mains entering the tank at the bottom, this water mixes with that at the bottom if their temperatures are within 0.5 °C of each other, otherwise a new segment is created. The system presented in this paper is simulated with TRNSYS using typical meteorological year (TMY) data for Nicosia, Cyprus. TMY is defined as a year, which sums up all the climatic information characterizing a period as long as the mean life of the system. The selection of typical weather conditions for a given location is very crucial in computer simulations for performance predictions and has led various investigators either to run long periods of observational data or to select a particular year, which appears to be typical from several years of data. The TMY for Nicosia, Cyprus, was generated from hourly measurements, of solar irradiance (global and diffuse on horizontal surface), ambient temperature, wind speed and
43
direction and humidity ratio, for a seven-year period, from 1986 to 1992 using the Filkenstein–Schafer statistical method (Kalogirou, 2003b). The measurements were recorded by the Cyprus Meteorological Service at the Athalassa region, an area at the suburbs of the town of Nicosia. The TMY is considered as a representative year for the Cypriot environment. 4.1. Economic analysis For this analysis, a life cycle analysis, in which the various costs are estimated annually, is considered. A life cycle analysis is performed in order to obtain the total cost (or life cycle cost) and the life cycle savings (LCS) of the system. This kind of economic analysis can be performed either within the TRNSYS environment or in a spreadsheet program (Kalogirou, 1996). The TRNSYS application is preferred here, as its output is given directly and avoids subsequent transfer of the thermal performance results to a spreadsheet. In general, the present worth (or discounted cost) of an investment or cost C at the end of year N, at a discount rate of d and interest rate of i is obtained by PWN ¼
Cð1 þ iÞN 1 ð1 þ dÞN
ð7Þ
In this method, the various costs and savings are estimated annually. From the addition of fuel savings incurred because of the use of the system and the tax savings, the mortgage, maintenance and parasitic costs are subtracted and thus the annual solar savings of the system are estimated which are converted into present worth values of the system. These are added up to obtain the life cycle savings according to the equation PWLCS ¼
N X Solar Savings N ð1 þ dÞ N ¼1
ð8Þ
The fuel savings are obtained by subtracting the annual cost of the conventional fuel used for the auxiliary energy from the fuel needs of a fuel only system. The integrated cost of the auxiliary energy use for the first year, i.e., solar backup, is given by the formula Z t C FA Qaux dt ð9Þ C aux ¼ 0
The integrated cost of the total load for the first year, i.e., cost of conventional fuel without solar, is Z t C FL Qload dt ð10Þ C load ¼ 0
where CFA and CFL are the cost rates for auxiliary energy and conventional fuel, respectively. In case that the same fuel is used for both CFA = CFL. The investment cost of the solar system in this case is estimated from the following equation:
44
S. Kalogirou / Solar Energy 83 (2009) 39–48
Cs ¼ Cf þ CaA
ð11Þ
Referring to the curve of hot water load (Qload), there is a decrease of hot water load demand during the summer months. This is attributed to the fact that during the summer months the total incidence solar radiation is higher which results to higher temperatures in the cold water storage tank. Consequently, the hot water demand from the hot water storage tank during these months is reduced. The variation of the annual solar contribution is also shown in Fig. 3. The solar fraction, f, is defined as the ratio of the useful solar energy supplied to the system to the energy needed to heat the water if no solar energy is used. In other words, f is a measure of the fractional energy savings relative to that used for a conventional system. It can be calculated from the following relationship:
where Cf is the collector area independent and Ca the collector area dependent costs. The values used here are Cf = 153 € and Ca = 128 €/m2. The maintenance cost (Cm), must also be considered. This is estimated to be 1% of the initial investment and is assumed to increase at a rate of 1% per year of the system operation. The total annual cost is given by C ¼ Cs þ Cm
ð12Þ
4.2. Results of the thermal and economic analysis Fig. 3 illustrates the monthly energy flows, which include the total radiation incident on the collector (Qins), the useful energy supplied from the collectors (Qu), the hot water energy requirements (Qload), the auxiliary energy demand (Qaux) and the heat losses from the storage tank (Qenv). As it can be seen from the graph of total radiation incident on collector (Qins), the maximum value occurs in the month of August (1.88 GJ). The useful energy supplied from the collectors (Qu) is maximized in the month of April (0.62 GJ). It can be also seen from Fig. 3 that there is a reduction in the incident solar radiation and consequently the useful energy collected during the month of May. This is a characteristic of the climatic conditions of Nicosia and is due to the development of clouds as a result of excessive heating of the ground and thus excessive convection, especially in the afternoon hours. The annual value of the useful energy supplied by solar energy (Qu) is equal to 6480 MJ. From the curve of the energy lost from storage tank (Qenv) it can be seen that during summer months the energy lost from storage to surroundings is maximized. This is due to the fact that at these months the temperature in the storage tank is higher and consequently more energy is lost.
f ¼
Qload Qaux Qload
ð13Þ
Fig. 3 implies that the solar fraction is lower in the winter months and higher, reaching 100%, in the summer months. The annual solar fraction is determined to be 79%. It should be noted that in domestic hot water systems, by adjusting slightly the consumption profile, contributions of 100% could be obtained in the months May–October, which is what actually happens in practice. The program however considers a standard consumption throughout all months that is why values slightly below 100% are given. The economic scenario used in this project is that all the cost of the solar system is paid from the beginning (i.e., no credit payments are assumed). The thermal performance degradation of the system is assumed to be 1% per year, the period of economic analysis is taken as 20 years (average life of locally produced systems), whereas all the other percentage figures (inflation rates and market discount rate) are mean values of the previous decade (Statistical abstracts, 2001). Electricity at a price of 0.153 €/kW h
2.00
1.00
1.80
0.90 0.80
ff Qins Qins Qu Qu Qload Qload Qaux Qaux Qenv Qenv
1.40 1.20 1.00 0.80
0.70 0.60 0.50 0.40
0.60
0.30
0.40
0.20
0.20
0.10
0.00
0.00 JAN
FEB
MAR
APR
MAY
JUN
JUL
AUG
SEP
OCT
NOV
Months
Fig. 3. Energy flows of the thermosiphon solar water heater.
DEC
YR
Solar Fraction
Energy Flows (GJ)
1.60
S. Kalogirou / Solar Energy 83 (2009) 39–48 Table 2 Results of economic analysis Output parameter
Electricity backup
Diesel backup
Initial cost of the system (€) Resale or salvage value (€) Rate of return of solar investment (%) Years until undiscounted fuel savings = investment Undiscounted cumulative net cash flow (€) Present worth of total costs with solar (€) Present worth of total costs without solar (€) Annualized total cost with solar (€/GJ) Annualized total cost without solar (€/GJ) Present worth of cumulative cash flow (€) Fuel price considered in the analysis
498 98 39.9 2.7
498 98 23.4 4.5
4616 1209 3450
2417 929 1985
18.7 54.4 2240 0.153 €/kW h
14.5 31.5 1056 0.76 €/l
and diesel at a price of 0.76 €/l are assumed to be used for auxiliary. Table 2 gives a summary of economic figures as calculated by TRNSYS. The results of the economic analysis shown in Table 2 were obtained by using the above (current) fuel and electricity rates, a 20-year period a market discount rate of 6% and interest rate of 8%. No subsidies were considered. As can be seen the solar system gives much lower specific energy cost than the conventional system for both types of auxiliary energy considered. The payback time is very low and is equal to two years and eight months for electricity backup and four years and six months for diesel backup. The life cycle savings represent the money that the owner will save by installing the solar system instead of buying electricity/fuel to satisfy his hot water needs and is equal to 2240 € for electricity backup and 1056 € for diesel backup. The difference in payback time and life cycle savings is due to the fact that in the case of diesel a fuel of lower price is replaced by the solar system. 5. Environmental benefits of thermosiphon solar water heating systems To investigate the environmental benefit of utilizing solar energy instead of conventional sources of energy, the different emissions resulting from the solar system operation are estimated and compared to those of a conventional electricity/fuel system. These are obtained by correlations derived by using program Polysun (2000) given in (Kalogirou, 2004b). The emissions reported are those which are responsible for the most important environmental problems as outlined in the previous sections. The environmental interventions are expressed in physical units of the emitted substances per year. The quantities of the emissions depend on the solar collector size and the required auxiliary energy and are compared to a non-solar system which is using conventional electricity/fuel. The environmental analysis of the system which includes the different pollutants as calculated by the program is tabulated in Tables 3 and 4 for electricity and diesel backup, respectively. In the tables the eight most
45
Table 3 Environmental impact of the thermosiphon solar water heater with electricity backup Emissions
Units
Conventional
Carbon dioxide (CO2) Carbon monoxide (CO) Nitrogen oxides (NOx) Nitrous oxide (N2O) Methane (CH4) Hydrocarbons Sulphur dioxide (SO2) Dust Savings in GHG
tons/ year g/year
374.6
109.7
70.7
g/year
56.3
16.3
71.1
g/year g/year g/year g/year
6.3 9.3 37.7 562.7
2.1 2.7 11.0 164.5
66.7 71.0 70.8 70.8
g/year %
188.1 –
54.8 –
70.9 70.3
1.546
Solar system 0.449
Savings (%) 70.1
Table 4 Environmental impact of the thermosiphon solar water heater with diesel backup Emissions
Units
Conventional
Solar system
Savings (%)
Carbon dioxide (CO2) Carbon monoxide (CO) Nitrogen oxides (NOx) Nitrous oxide (N2O) Methane (CH4) Hydrocarbons Sulphur dioxide (SO2) Dust Savings in GHG
tons/ year g/year
0.889
0.293
67.1
1688
581.3
65.6
g/year
1636
544.8
66.7
g/year g/year g/year g/year
6.1 13.6 52.7 651.4
1.2 3.3 12.9 169.9
80.0 75.7 75.5 73.9
g/year %
180.0 –
73.5 –
59.2 70.5
important greenhouse gasses are considered. For the case of electricity backup (Table 3), Polysun considers a mixture of European power stations (coal-based, nuclear, hydroelectric, etc.) in order to estimate the emissions of the conventional system (Polysun, 2000). As can be seen in both cases by using solar energy instead of conventional fuel very large amount of pollutants are avoided. The amount of emissions depends on the type of fuel used as auxiliary. The percentage saving obtained in the cases where electricity or diesel backup is used is about 70%. It should be noted however that the quantities of emissions in the various substances emitted are completely different and the proximity of the total percentage numbers obtained is due to the generation efficiency of each system. Electrical energy is produced at a maximum efficiency of about 35% whereas in the case of diesel backup a boiler efficiency of 85% is considered. 6. Pollution created from thermosiphon solar water heating systems The negative environmental impact of any solar energy system includes land displacement, and possible air and
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S. Kalogirou / Solar Energy 83 (2009) 39–48
water pollution resulting from the manufacture, normal maintenance operations and demolition of the systems. However, land use is not a problem when collectors are mounted on the roof of a building, maintenance required is minimal and pollution caused by demolition is not greater than the pollution caused from demolishing a conventional system of the same capacity, and the great majority of the solar system components can be recycled, therefore not disposed to the environment. The pollution created for the manufacture of the solar collectors is estimated by calculating the embodied energy invested in the manufacture and assembly of the collectors and estimating the pollution produced by this energy. Initially, the embodied energy of one solar collector panel, 1.35 m2 in area is determined. This is the same collector considered in the performance analysis of the system. The analysis is based on the primary and intermediate embodied energy of the components and materials as illustrated in Fig. 4. In the present analysis no allowance is made for the unit packing, transportation and maintenance as these have insignificant contribution compared to the total. The total embodied energy required to produce a complete flat-plate collector is calculated using primary and intermediate production stages. The primary stage is established from an assessment of the various materials used and their corresponding mass. Using the embodied energy index (MJ/kg) defined by Alcorn (1995) the material embodied energy content within the unit is determined. Table 5 summarizes the unit materials used and lists their corresponding mass and embodied energy content. The total embodied energy content for the production of one flat-plate collector panel is calculated at 2663 MJ. This comprise the primary embodied energy of materials and the intermediate embodied energy, i.e., the amount of
PRIMARY PRODUCTION
Table 5 Embodied energy content of one flat-plate collector 1.35 m2 in area Description
Mass (kg)
Embodied energy index (MJ/kg)
Embodied energy content (MJ)
1.6 0.85 0.05 m insulation 1.6 0.85 0.005 m glass 1.8 m, 22 mm copper pipe 16 m, 15 mm copper pipe 2 1.05 0.005 m galvanized steel sheet 5 m rubber sealant Black paint Casing paint 20 No. screws 169 0.85 0.003 m copper absorber
4.3
117
503.1
9.5
15.9
151.1
2.16
70.6
152.5
9.9
70.6
698.8
8.2
34.8
285.4
0.5 0.3 0.9 0.00125 3.6
110 44 44 34.8 70.6
55 13.2 39.6 Ignored 254.2
Total Add 10% for contingencies Unit manufacture using a net to gross value of conversion rate of 27%
2153 215 295
Grant total
2663
energy used in the production and assembly of the component parts during the construction stage and was determined through a stage-by-stage appraisal of the power sources used. Inherent within this intermediate stage is the fabrication of purchased components like screws, glass and insulation. An analysis of the embodied energy content of a complete thermosiphon solar water heating system is shown in Table 6. It should be noted that only the extra compo-
INTERMEDIATE PRODUCTION
Paint, sealant, coatings Primary raw materials extraction and production
Glass Copper pipes
Absorber
Copper Sheet Insulation Galvanized sheet
Casing
Packing Transportation Installation Maintenance Demolition Disposal/recycling
Fig. 4. Factors considered in the calculation of embodied energy of a flat-plate collector.
S. Kalogirou / Solar Energy 83 (2009) 39–48 Table 6 Embodied energy content for the construction and installation of the complete thermosiphon solar water heating system Description 2 No. solar panels 4 m, 22 mm copper pipe 4 m, pipe insulation Steel frame
Mass (kg) – 3.8 1 30
Embodied energy index (MJ/kg) – 70.6 120 34.8
Embodied energy content (MJ)
Table 7 Pollution created for the construction and installation of the thermosiphon solar water heating system and payback for the two types of backup fuels considered Emission
Pollution created from solar system embodied energy
Savings and payback of solar system Electricity
Diesel
Carbon dioxide (CO2) Carbon monoxide (CO) Nitrogen oxides (NOx) Nitrous oxide (N2O) Methane (CH4) Hydrocarbons
1.9 tons
1.097 (1.7)
0.596 (3.2)
460.1 g
264.9 (1.7)
1106.7 (0.4)
69.2 g
40 (1.7)
1091.2 (0.06)
6.3 g
4.2 (1.5)
11.4 g
6.6 (1.7)
46.3 g
26.7 (1.7)
691.2 g
398.2 (1.7)
4.9 (1.3) 10.3 (1.1) 39.8 (1.2) 481.5 (1.4)
231.1 g
133.3 (1.7)
106.5 (2.2)
5326 268.3 120 1044
Total Installation
6758.3 187.7
Grant total
6946
nents of the solar system are considered in this analysis as the other components are standard and are also present in conventional systems. Here, the objective is to compare the pollution created for the manufacture and installation of the solar system against its benefits due to the lower emissions realized during the operation of the system. As can be seen the total embodied energy for the complete system is 6946 MJ. For the life cycle assessment of the system considered the useful energy supplied by solar energy per year, shown in Fig. 3 (6480 MJ) is compared with the total embodied energy of the system shown in Table 6. As can be seen the total energy used in the manufacture and installation of the system is recouped in about 13 months which is considered as very satisfactory. The emissions created from total embodied energy are presented in Table 7. Additionally, these emissions are compared with the emissions produced from the auxiliary energy, according to the type of fuel used in the various cases investigated, in order to estimate the payback period for each pollutant. In all cases the emissions are estimated by considering that all embodied energy was produced from electricity. This is not quite correct but electricity is chosen, as is the most polluting fuel, therefore it gives the worst possible results. As can be seen from Table 7, the payback periods for the cases investigated vary from a few months to 3.2 years according to the fuel and the particular pollutant considered. The size of thermosiphon solar water heater considered is the usual type encountered in Cyprus. Cyprus began manufacturing solar water heaters in the early sixties. Today more than 93% of all houses have solar water heating systems installed and operating. The total number of systems is equal to 190,000 units. In fact the number of units in operation today corresponds to one heater for every 3.7 people in the island, which is a world record (Kalogirou, 2003a). If we consider that all systems are of the same size as the one investigated here and that 50% of the systems are using electricity as backup and 50% diesel then the amount of pollutants avoided per year are as shown in Table 8. As can be seen considerable environmen-
47
Sulphur dioxide (SO2) Dust
Note: (1) Number in parenthesis represent payback time in years. (2) The units of savings are in g/year except carbon dioxide which is tons/year.
Table 8 Annual environmental pollution reduction because of the use of thermosiphon systems in Cyprus Emission
Savings
Carbon dioxide (CO2) Carbon monoxide (CO) Nitrogen oxides (NOx) Nitrous oxide (N2O) Methane (CH4) Hydrocarbons Sulphur dioxide (SO2) Dust
160,835 tons 130.3 tons 107.5 tons 864.5 kg 1605.5 kg 6317.5 kg 83.6 tons 22.8 tons
Notes: 1. Total number of units considered = 190,000. 2. Savings estimated by considering that 50% of the systems are using electricity and 50% diesel as backup.
tal pollution reduction occurs each year just for water heating. Moreover, the cost of damage avoided by some of the pollutants is investigated with respect to damages to crops, materials, mortality (refers to premature deaths) and morbidity (refers to illness). The results of this analysis are shown in Tables 9 and 10. As can be seen 36 € are avoided per year when the system is using electricity as auxiliary and 22.4 € when diesel is used for each thermosiphon solar water heating system. Therefore for a more correct analysis of solar systems the damage cost avoided, shown in Table 10, should be added to the annual fuel savings which will reduce the payback time even more, thus there is a further
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S. Kalogirou / Solar Energy 83 (2009) 39–48
Table 9 Typical damage costs per kilogram of pollution emitted by power plants in Europe (Rabl and Spardaro, 2001) Pollutant
Impact
Cost (€/kg)
SO2 CO (primary) CO2
Crops, materials, mortality and morbidity Morbidity Global warming
10.55 0.002 0.029
Table 10 Damage cost avoided per year from some of the pollutants for thermosiphon solar water heating systems Pollutant
CO2 CO SO2
Amount saved (kg)
Damage cost avoided (€)
Electricity
Diesel
Electricity
Diesel
1097 0.265 0.398
596 1.107 0.482
31.8 0 4.2
17.3 0 5.1
36
22.4
Total
increase in the economic viability of the systems. It is believed by the author that such type of analysis must always be considered in feasibility studies of solar systems. 7. Conclusions In the present study, the potential benefits that solar systems offer are discussed in detail. From the analysis presented in this paper it can be concluded that the environmental impact of any energy system is an important factor and solar systems have the potential to reduce environmental pollution. Additionally, in this study the environmental protection offered by the most widely used renewable energy system, i.e., the thermosiphon solar water heating system is presented. The results show that by using solar energy considerable amounts of greenhouse polluting gasses are saved. For the domestic solar heating system considered here, with electricity or diesel backup the saving, compared to a conventional system, is about 70%. Additionally, the system investigated give positive and very promising performance and financial characteristics. The annual solar contribution is 79% whereas the payback time of the system is 2.7 year and the life cycle savings are 2240 € for electricity backup and 4.5 years and 1056 € for diesel backup, respectively. With respect to life cycle assessment of the systems, the energy spent for the manufacture and installation of the solar systems is recouped in about 13 months, whereas the payback time with respect to emissions produced from the embodied energy required for the manufacture and installation of the systems varies from a few months to 3.2 years according to the fuel and the particular pollutant considered. Moreover, the cost of damage avoided by some of the pollutants is investigated with
respect to damages to crops, materials, mortality and morbidity and found that 36 € are avoided per year when the system is using electricity as auxiliary and 22.4 € when using diesel. It can therefore be concluded that solar energy systems are efficient, cost effective and friendlier to the environment. The reduction of greenhouse gasses pollution is the main advantage of utilizing solar energy. Therefore, solar energy systems should be employed whenever possible in order to achieve a sustainable future. References Alcorn, J., 1995. Embodied energy coefficients of building materials. Centre for Building Performance Research, Victoria University of Wellington, New Zealand. Argiriou, A., Klitsikas, C., Balaras, C., Asimakopoulos, D., 1997. Active solar space heating of residential buildings in northern Hellas – a case study. Energy and Buildings 26 (2), 215–221. Diakoulaki, D., Zervos, A., Sarafidis, J., Mirasgedis, S., 2001. Cost benefit analysis for solar water heating systems. Energy Conversion and Management 42 (14), 1727–1739. Fanney, A.H., Klein, S.A., 1983. Performance of solar domestic hot water systems at the National Bureau of Standards – measurements and predictions. ASME Solar Energy Engineering 105, 311–321. Hasan, A., 2000. Optimisation of collector area in solar water heating systems. International Journal of Solar Energy 21 (1), 19–27. Kalogirou, S., 1996. Economic analysis of solar energy systems using spreadsheets. In: Proceedings of the World Renewable Energy Congress IV, Denver, CO, USA. vol. 2. pp. 1303–1307. Kalogirou, S., 2003a. The energy subsidisation policies of Cyprus and their effect on renewable energy systems economics. Renewable Energy 28 (11), 1711–1728. Kalogirou, S., 2003b. Generation of typical meteorological year (TMY-2) for Nicosia, Cyprus. Renewable Energy 28 (15), 2317–2334. Kalogirou, S., 2004a. Solar thermal collectors and applications. Progress in Energy and Combustion Science 30 (3), 231–295. Kalogirou, S., 2004b. Environmental benefits of domestic solar energy systems. Energy Conversion and Management 45 (18–19), 3075–3092. Keyanpour-Rad, M., Haghgou, H.R., Bahar, F., Afshari, E., 2000. Feasibility study of the application of solar heating systems in Iran. Renewable Energy 20 (3), 333–345. Mirasgedis, S., Diakoulaki, D., Assimacopoulos, D., 1996. Solar energy and the abatement of atmospheric emissions. Renewable Energy 7 (4), 329–338. Rabl, A., Spardaro, J.V., 2001. The cost of pollution and the benefit of solar energy. In: Gordon, J.M. (Ed.), Solar Energy: The State of the Art ISES. James and James Ltd., London, pp. 437–475, (Chapter 8). Polysun, 2000. User’s manual for Polysun 3.3. SPF, Switzerland. Statistical abstract, 2001. Statistical service. Republic of Cyprus, Report No. 44. Taborianski, V.M., Prado, R.T.A., 2004. Comparative evaluation of the contribution of residential water heating systems to the variation of greenhouse gases stock in the atmosphere. Building and Environment 39 (6), 645–652. TRNSYS, 2005. Version 16 program manual. Solar Energy Laboratory, University of Wisconsin, Madison, USA. Tsillingirides, G., Martinopoulos, G., Kyriakis, N., 2004. Life cycle environmental impact of a thermosiphon domestic solar hot water system in comparison with electrical and gas heating. Renewable Energy 29 (8), 1277–1288.